Automotive body panels and sheet metal products have been made of suitable steel alloys by stamping processes at ambient temperatures. The edges of a steel sheet blank are gripped by a binder mechanism and a punch pushes and draws the metal against a generally concave forming surface. Often a steel alloy is available that is suitably formable and the metal is stretched into a complex shape such as a body panel without tearing, wrinkling or otherwise marring the sheet. A progressive sequence of stamping and, for example, piercing and trimming operations may be successively performed on a single steel sheet to make a panel with each step being completed in a matter of seconds.
Aluminum sheet alloys would be substituted for steel in many applications to save weight. For example, some Aluminum Association alloys of the 1×××, 3××× and 5××× series have been used in stamping operations. But such aluminum stamping alloys are not as ductile and formable as steel alloys and the aluminum often tears if it is stamped to the same shape. The aluminum alloy work-hardens at stamping strain rates and some portion of the sheet yields and tears. This property of aluminum alloys has limited the product shapes to which they can be formed by high production rate stamping. Complex panel shapes often have to be made in multiple pieces and welded together. This usually results in higher manufacturing cost and may complicate dimensional control of the composite product.
It is an object of this invention to provide a method for using work-hardenable aluminum sheet alloys in relatively high production rate stamping operations to form one-piece products whose shape includes areas of deformation that exceed the strain limits of the starting sheet material.